Field of the Invention
The present invention relates to an auto-focus adjustment device of an imaging apparatus. In particular, the present invention relates to an auto-focus adjustment technique suitable for use in cameras or the like having a focus adjustment function using an imaging plane phase difference detection method.
Description of the Related Art
There have been proposed auto-focus adjustment (AF) devices that perform pupil division-type phase difference detection using a pair of photoelectric conversion units provided in each microlens in two-dimensional array. The AF device of this type detects a positional phase difference between two light fluxes (between object images) transmitted through different portions of the pupil of an imaging optical system and converts the positional phase difference into the amount of defocus. In general, a focus lens is driven each time the amount of defocus is detected until the focus is adjusted on an object. This is because a conversion coefficient used when the amount of defocus detected is converted into a pulse as the drive amount for a lens differs for each focus position. In other words, the AF device acquires a conversion coefficient at current focus position and then updates the number of drive pulses so as to obtain an in-focus state. As compared with the in-focus state, two object images blur in an out-of-focus state. Thus, the amount of defocus cannot be detected or when the amount of defocus includes an error, an in-focus state cannot be obtained by moving a lens after one-time detection of the amount of defocus. Defocus amount detection and focus adjustment needs to be performed by periodically acquiring two object images.
In the camera including a phase difference detection-type AF device, exposure control is typically performed so as to obtain a brightness signal suitable for use in focus adjustment but is performed with the focus lens stopped. In other words, in the phase difference detection method for repeatedly driving a focus lens until the focus lens is finally located in the in-focus position after periodical detection of the amount of defocus, exposure control is performed prior to start of driving of the lens. In the method, the following phenomenon occurs in a state where an object image is largely blurred. In particular, in the case of a high brightness object, the brightness level of the object becomes relatively high when the focus is on the object to some extent. However, when the focus is being adjusted on the object while performing exposure control in a state where the object image is largely blurred, the level of the brightness signal obtained from the imaging element becomes too high and reaches a saturated state. When the contrast of an object to which the focus wants to be adjusted cannot be appropriately evaluated due to saturation, erroneous detection of the focus state occurs, so that the focus lens cannot be in an in-focus state or in a largely-defocused state.
In order to counteract this phenomenon, Japanese Patent Laid-Open No. H1-157173 discloses a method for not using a focus area determined to be a high brightness object and its peripheral focus areas from among a plurality of focus areas. Japanese Patent Laid-Open No. 2006-215391 discloses a method for not performing focus adjustment when the proportion of the area within a focus area in a range where the brightness level is saturated to the area of the focus area is equal to or greater than a predetermined level.
However, in the conventional technique disclosed in Japanese Patent Laid-Open No. H1-157173, when the high brightness object exists on the selected focus area, an area other than the focus area is selected. In the conventional technique disclosed in Japanese Patent Laid-Open No. 2006-215391, when the selected focus area corresponds to the high brightness object, the focus cannot be reached at the focus area. When the brightness of the object is high, a result different from capturing a focused image in a composition as intended by a photographer may be obtained.
In some image-taking lenses (so-called macrolens) for close-up photography, an effective open F-number may rapidly change when the lens is extended. For example, when the focus is adjusted on the object corresponding to a macro area from the outside of the macro area, it is assumed that the brightness of the object within the focus area is appropriately obtained by exposure control prior to start of driving of the focus lens. However, the level of the brightness signal obtained from the imaging element becomes low when the lens is extended, resulting in deviation from appropriate exposure. When the contrast of the object to which the focus wants to be adjusted cannot be appropriately evaluated under such a condition, erroneous detection of the focus state occurs, so that the focus lens cannot be in an in-focus state or in a largely-defocused state.